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Quantum Electronics and Laser Science Conference

Observation of Atomic Wannier-Stark Ladders

C. F. Bharucha, Kirk W. Madison, Qian Niu, Steven R. Wilkinson, Mark G. Raizen

Published Jun 2, 1996• 0 citations
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Abstract

We report the first observation of atomic Wannier-Stark ladders in a potential formed by an accelerating standing wave of light.1 This is a fundamental effect of quantum transport in optical lattices. Wannier-Stark ladder resonances were predicted to occur in the band structure of an electron moving in a periodic lattice with a dc electric field, and have been recently observed in superlattices. Our work is the first spectroscopic study of Wannier-Stark ladders free from complications such as the impurities, lattice vibrations, and multi-particle interactions present in solids.

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We report the first observation of atomic Wannier-Stark ladders in a potential formed by an accelerating standing wave of light.1 This is a fundamental effect of quantum transport in optical lattices. Wannier-Stark ladder resonances were predicted to occur in the band structure of an electron moving in a periodic lattice with a dc electric field, and have been recently observed in superlattices. Our work is the first spectroscopic study of Wannier-Stark ladders free from complications such as the impurities, lattice vibrations, and multi-particle interactions present in solids.
We report an experimental study of the Wannier-Stark fan and the fractional Wannier-Stark ladder using laser-cooled sodium atoms in an accelerating one-dimensional standing wave of light. We prepare the atoms in the lowest motional band of the optical lattice and then impose a constant acceleration. A weak oscillatory component is added to the acceleration in order to resonantly drive interband transitions, and the number of atoms that remain in the lowest band is measured as a function of the probe frequency. The spectrum is characterized by a ladder of resonances spaced by the atomic Bloch oscillation frequency ${\ensuremath{\omega}}_{B}.$ When an additional, strong ac component at frequency $\ensuremath{\Omega}$ is added, a fractional ladder is observed with a spacing related to the electric matching ratio $\ensuremath{\Omega}/{\ensuremath{\omega}}_{B}.$
We report observation of Wannier-Stark ladders with ultracold sodium atoms in an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. A small oscillatory component is added to the acceleration, and the fraction of trapped atoms is measured as a function of the oscillation frequency. Resonances are observed where the number of trapped atoms drops dramatically. The separation between resonant peaks is found to be proportional to the acceleration. We show that this resonant structure can also be understood as a temporal quantum interference effect.
The electric field intensity of a standing-wave laser field is periodic in space. Due to the ac Stark effect, this gives rise to a spatially periodic potential acting on an atom, as explained in Chapter 4 (see, e.g., Eq. (4.31)). This is the physical principle behind the generation of optical lattices. By superimposing a number of different laser beams it is possible to generate potentials which are periodic in one, two or three dimensions. The suggestion that standing light waves may be used to confine the motion of atoms dates back to 1968 and is due to Letokhov. The first experimental realization of an optical lattice was achieved in 1987 for a classical gas of cesium atoms. The study of atoms in such potentials has many different facets. At the simplest level, it is possible to study the energy band structure of atoms moving in these potentials and to explore experimentally a number of effects that are difficult to observe for electrons in the periodic lattice of a solid. Interactions between atoms introduce qualitatively new effects. Within mean-field theory, which applies when the number of atoms in the vicinity of a single minimum of the potential is sufficiently large, one finds that interatomic interactions give rise to novel features in the band structure. These include multivaluedness of the energy for a given band and states possessing a periodicity different from that of the optical lattice.